1. Field of the Invention
The present invention relates generally to a method and system for detecting or deriving wheel slippage in an automotive vehicle. More specifically, the invention relates to a method and a system for deriving a wheel slippage value to utilize in traction control for automotive vehicle wheels, in an anti-skid brake control system and so forth.
2. Description of the Background Art
In general, wheel slippage is reflected in the difference between actual vehicle speed and an assumed vehicle speed derived based on previous wheel speeds. When the actual vehicle speed is lower than the assumed vehicle speed, it means the driven wheel is spinning. On the other hand, when the actual vehicle speed is higher than the assumed vehicle speed, it means the vehicle wheel is skidding. Wheel-spin is caused by loss of road/tire traction. Therefore, in such cases, traction must be controlled in order to stop the vehicular wheel from spinning. On the other hand, wheel-skidding occurs during abrupt application of the brakes and is due to the wheels locking up. For example, such traction controls have been disclosed in U.S. Pat. No. 3,893,535, issued on July 8, 1975, to M. H. Burckhardt, et al. and in Japanese Patent First Publication (Tokkai Showa) 59-68537, published on Apr. 18, 1984. In both cases, the rotation speed of a driven wheel which is driven by engine output is compared with the rotation speed of a non-driven wheel which rotates freely. The rotation speed of the non-driven wheel is treated as a parameter reflecting actual vehicle speed.
The detection of the wheel slippage as disclosed in the aforementioned prior art is not applicable to anti-skid or traction control for a four-wheel drive vehicle. This is due to the fact that a four-wheel drive vehicle has four driven wheels and thus there are no non-driven wheels free of engine output.
On the other hand, a method has been proposed for utilizing a projected vehicle speed value which is derived from the wheel speed obtaining immediately before the wheel slippage occurs. In this case, instantaneous projected vehicle speed at every moment is assumed based on the projected vehicle speed derived based on the wheel speed immediately before the wheel slippage occurs, according to a predetermined vehicle speed variation pattern.
On the other hand, vehicle speed variation is seriously affected by road surface friction .mu., Therefore, the rate of change of vehicle speed varies with the road surface friction. However, in the conventionally proposed method, the vehicle speed variation pattern is a fixed pattern regardless of possible changes in the road surface friction .mu.. Therefore, the instantaneous projected vehicle speed does not always reflect the actual vehicle speed.